An ink-jet printer includes a pen in which small droplets of ink are formed and ejected toward a printing medium. Such pens include print heads with orifice plates having very small nozzles through which the ink droplets are ejected. Adjacent to the nozzles (inside the print head) are ink chambers, where ink is stored prior to ejection. Ink is delivered to the ink chambers through ink channels that are in fluid communication with an ink supply. The ink supply may be, for example, contained in a reservoir part of the pen.
Ejection of an ink droplet through a nozzle may be accomplished by quickly heating a volume of ink within the adjacent ink chamber. The rapid expansion of ink vapor forces a portion of the ink in the chamber through the nozzle in the form of a droplet. This process is called "firing." The ink in the chamber is heated with a heat transducer that is aligned adjacent to the nozzle. Typically, the heat transducer is a resistor, or piezoelectric transducer. Such printers are known as thermal ink-jet printers.
Thin-film resistors are typically used in print heads of thermal ink-jet printers. In such a device, a resistive heating material is typically disposed on a silicon substrate. Conventional fabrication techniques allow placement of a substantial number of resistors on a silicon wafer substrate.
In the past, the number of resistors applied to the silicon substrate was limited by the conductive components used to electrically connect the print head to external pulse driver circuitry for selectively heating the resistors. Thus, thermal inkjet print heads have been developed which incorporate pulse driver circuitry directly on the print head silicon substrate with the resistors. The incorporation of the pulse driver circuitry on the print head silicon substrate reduces the number of interconnect components needed to electrically connect the pen to the printer, thereby allowing fabrication of smaller ink-jet pens.
The pulse driver circuitry located on the silicon substrate typically comprises MOSFET drive transistors (i.e., metal oxide semiconductor field effect transistors). The integrated circuitry (i.e., resistors, transistors, and interconnects) is dimensionally defined on the substrate by conductive trace patterns, lithographically formed using conventional masking, ultraviolet exposure and etching techniques known in the art. Accordingly, in typical ink-jet print heads, the silicon substrate provides (1) mechanical support for the electronic components formed thereon, (2) acts as a thermal conductor, and (3) forms a part of the electronic device (i.e., a channel of the MOSFET transistor is formed in (or "by") the silicon substrate).
The pulse driver circuitry (hereafter referred to as a print head) is affixed adjacent to an ink barrier and an outer orifice plate. The internal geometry of the ink barrier defines the shape of the ink chamber. The ink chamber is situated above, and aligned with, a corresponding resistor which, when heated, ejects a droplet through the chamber nozzle. The integration of driver components and heater resistors on the same print head substrate requires multi-layer connective circuitry so that the driver transistors can communicate with the resistors and other portions of the printing system.
The amount of energy necessary for ejection of ink droplets from the chamber is known in the art as "turn on energy" or TOE. A higher TOE may result in excessive print head heating. Excessive print head heating generates bubbles from air dissolved in the ink and causes the ink vapor bubble to form prematurely. Air bubbles within the ink and premature formation of the vapor droplet result in poor ink droplet formation and, thus, poor print quality. Print speed must be slowed to a rate that prevents excessive print head heating. Accordingly, the thermal conductivity of the underlying substrate factors significantly in the control of ink droplet formation and firing characteristics of the print head.
The present invention provides an integrated, thin-film print head for a thermal ink-jet printer. The driver circuitry includes MOSTFT transistors (i.e., metal oxide semiconductor thin film transistors). Accordingly, the print head is manufactured on a relatively inexpensive, non-silicon substrate or a less expensive silicon substrate, due to the less restrictive criteria for the substrate material required by MOSTFT transistors. Further, the non-silicon substrate may comprise a material that provides enhanced thermal conductivity. A relatively thermally conductive substrate acts as a heat sink for the print head, thereby reducing print head heating. Reduction of print head heating allows the printer to operate at higher speeds without degradation of the print quality.
More specifically, the thin-film, ink-jet print head of the present invention includes at least one MOSTFT transistor, a corresponding resistor, and interconnections for electrical communication between the components. Because the print head comprises MOSTFT transistors, unlike conventional print heads having MOSFET transistors, channels are not formed in the substrate. As a result, the requirements of the substrate material are considerably reduced, allowing use of relatively inexpensive substrate material and simplifying the manufacturing process.
For example, because a MOSTFT transistor channel is not formed in the substrate, the manufacturing process of the present print head does not require controlled doping of the substrate or that the substrate have a specific oxygen content, as is required for the manufacture of conventional print head substrates. Further, in conventional MOSFET transistor print heads, the transistor channel was formed in the substrate and hence was "tied" to the body of the print head. As a result, each transistor channel was not necessarily electrically isolated from an adjacent transistor. Formation of the channel in the substrate thus frequently resulted in some leakage of current through the body, causing a change in the threshold voltage of neighboring transistors. This phenomena is known to those persons skilled in the art as "body effect."
Because the transistor channel of the present print head is not formed in the substrate, each transistor channel is effectively isolated from neighboring electronic components. Accordingly, the present print head avoids poor print head operation caused by body effects in conventional print heads.